The present invention relates to a multi-scan recording method and a high speed recording apparatus, which can be used in combination to record at a high speed and a high level of quality, without the problems which occur when a conventional multi-scan recording method is used in combination with a high speed recording apparatus.
In recent years, various office automation business machines such as personal computers or word processors have come to be widely used, along with various recording apparatuses for printing out the data inputted with the use of these machines. Also, the technologies for printing high quality images at a high speed with the use of these machines have been rapidly developed.
First, these technologies will be briefly described.
(1) Technologies for Improving Image Quality
One of the technologies for improving image quality is a multiple scanning recording.
When an image is recorded with the use of a recording head comprising a plurality of recording elements, the quality of an image to be recorded greatly depends on the performance of the recording head itself. Slight differences in shape among the ejection orifices of a recording head or electrothermal transducers (ejection heaters), which occur during the recording head manufacturing process, affect the amount by which ink is ejected from each ejection orifice, or the direction in which ink is ejected from each ejection orifice. In other words, the presence of these differences manifest as nonuniform density in a final image, reducing the image quality.
An example of such manifestation of nonuniform density will be described with reference to FIGS. 1 and 2. In FIG. 1, (a), a referential character 201 designates a recording head. For the sake of more simple description, it is assumed that the recording head 201 comprises eight nozzles 202. A referential character 203 designates an ink droplet ejected by a nozzle 202. Ideally, all ink droplets are the same in terms of the amount (normal amount) by which they are ejected, and also in terms of the direction in which they are ejected as illustrated in FIG. 1, (a). When ink is ejected as illustrated in FIG. 1, (a), all the dots formed on a piece of recording sheet as the ink droplets land on the recording sheet will be uniform in size as illustrated in FIG. 1, (b). Therefore, an image which does not suffer from nonuniform density, as illustrated in FIG. 1, (c), is formed.
In reality, however, each nozzle 202 of the recording head 201 is different in ejection characteristic from the other. Therefore, if an image is recorded as described above, the ink droplet ejected from each nozzle becomes different in size and direction from those ejected from the other nozzles as illustrated in FIG. 2, (a). As a result, a pattern such as the one illustrated in FIG. 2, (b) is created on a piece of recording sheet as the ink droplets land on the recording sheet. In other words, as is evident from FIG. 2, (b), white spots are visible, which are created when the area factor fails to reach 100%, that is, when a unit strip of recording field fails to be 100% covered with the ink. These white spots align in the primary scanning direction of the recording head. Also, there are spots where the dots excessively overlap, and/or white strips like the white strip at the center of FIG. 2, (b). In other words, the recording density distribution of the image created as ink droplets different in size and direction land on the recording sheet becomes as illustrated in FIG. 2, (c), being nonuniform in terms of the direction in which the nozzles are aligned. This nonuniform density distribution is sensed as an anomaly by a person with normal eyesight.
Thus, the following method has been devised as a measure for dealing with such a problem, which will be described with reference to FIGS. 3 and 4. Referring to FIG. 3, according to this method, in order to complete an image area illustrated in FIGS. 1 and 2, the recording head 201 was run three times. The top half of the image area, which amounts to one half of the image area, and corresponds to a unit of four picture elements, is completed through two runs of the recording head 201. More specifically, the eight nozzles of the recording head 201 are divided into top and bottom groups, which comprises the top four nozzles and the bottom four nozzles, respectively. The number of dots placed per scan, or recording run, by a single nozzle is half the number of the dots, which will be placed in accordance with a full set of image formation data correspondent to a single raster when a conventional multi-scan recording method is employed; in other words, the dot density is thinned to a half, according to a predetermined image data arrangement. During the second run, dots are filled into the spots correspondent to the remaining half of the image formation data, to complete the aforementioned strip of recording field correspondent to a unit of four recording elements. This recording method is called the xe2x80x9cmulti-scan recording methodxe2x80x9d.
With the use of this recording method, the effects of the difference in size and direction among the nozzles of a recording head is halved even when a recording head such as the one illustrated in FIG. 2 is used. Therefore, an image area such as the one illustrated in FIG. 3, (b) is created, in which the black or white strips illustrated in FIG. 2, (b) are less obtrusive, and which has a recording density distribution illustrated in FIG. 3, (c); anomaly in density is substantially small.
Also in this recording method, a set of image formation data is divided according to a predetermined arrangement (mask), into two sub-sets, which compensate for each other. One of the most commonly used image formation data arrangement patterns (dot density thinning pattern) is a pattern which produces the dot placement pattern illustrated in FIGS. 4, (a)-(c), in which the dot density is thinned by eliminating every other dot in terms of both the vertical and horizontal directions so that the remaining dots are staggered relative to the dots in the adjacent lines. A unit strip of recording field (recorded here by a unit of four recording elements) is recorded by two recording runs: the first run which places dots in the staggered pattern illustrated in FIG. 4, (a), and the second run which places dots in the staggered pattern illustrated in FIG. 4, (b). The second staggered pattern is the reversal of the first staggered pattern.
In most cases, the distance recording medium is advanced in the secondary scanning direction, per recording run, is set to be constant. In the case illustrated in FIGS. 3 and 4, recording medium was advanced by a distance equivalent to four nozzles, per recording run.
(2) High Speed Recording Technology
As one example of technology for increasing recording speed, it is possible to increase the number of nozzles. In the case of the serial recording method in which a unit strip of recording field is recorded through a single recording run in which all the nozzles of a recording head are activated, recording speed increases as the number of nozzles is increased, although the increase in recording speed is not exactly proportional to the increase in the number of nozzle because of the time used for feeding or discharging a plurality of recording sheets, or the like operations. For example, if a recording head with 64 nozzles is set to record at a resolution of 360 dpi, the recording of a piece of recording medium with a size of A4 can be completed by approximately 60 recording runs. However, if a recording head with 256 nozzles is set to record at the same resolution, the recording of the same medium can be completed by approximately 15 recording runs; in other words, recording speed increases to a speed four times the speed of the first head.
In the case of the first head, the length of the nozzle alignment is approximately 4.52 mm (=25.4 mm/360 dpixc3x9764 nozzles), whereas in the case of the second head, it is 18.06 mm (25.4 mm/360 dpixc3x97250 nozzles), being approximately four times that of the first head.
At this time, formation of an illusionary line will be described. It is assumed that the state of ink ejection from a recording head, and the landing position of each ink droplet, and the like factors, are ideal.
In the case of an ink jet recording system, a dot is often placed so that it overlaps with a priorly formed dot. In the area where the two dots overlap, the ink of the second dot, or the dot placed later, tends to penetrate deeper into the recording medium than the ink of the dot placed first.
FIG. 5 is a schematic sectional view of a piece of recording medium P, and two ink droplets; the first ink droplet, and the second ink droplet ejected following the first. It sequentially depicts how the two ink droplets behave after hitting the recording medium. As the ink comes in contact with the recording medium P, coloring component, such as dye, in the ink combines with the recording medium P, physically as well as chemically. However, since there is a limit to the amount by which coloring component can combine with the recording medium P, the combination of the coloring component in the first ink droplet and the recording medium P takes precedence in terms of bonding strength unless there is a substantial difference in bonding strength between the coloring components in the first and second ink droplets. Therefore, it is relatively difficult for the coloring component in the second ink droplet to combine with the portion of the recording medium P close to the recording surface. As a result, the coloring component in the second ink droplet penetrates deeper into the recording medium P, coloring the deeper portion of the recording medium P. Further, taking into consideration the ink behavior at the level of the fiber in the recording medium P, a piece of fiber having once bonded with the dye or the like in the ink is stronger in terms of hydrophilicity than the one having never bonded with the dye. Therefore, there is a tendency that if an ink droplet lands adjacent to the portion of the recording medium P stronger in hydrophilicity, the ink from the ink droplet is drawn toward the portion of the recording medium P where the preceding ink droplet landed.
This results in the following problems. Referring to FIG. 6, as recording is made immediately adjacent to the area in which recording has been completed, ink droplets land at the border between the recorded and unrecorded areas, and the ink of the ink droplets is drawn into the priorly recorded area. As a result, the portions of the priorly recorded area immediately adjacent to this border are supplied with an additional amount of ink, increasing in coloring component density When this phenomenon occurs to a limited number of the portions along the border, these portions do not stand out. However, when these portions with a higher color component density align in the raster direction, the aligned portions are perceived as a single line, or xe2x80x9cillusionary linexe2x80x9d.
When a technology is considered for improving image quality, along with a technology for increasing recording speed, the illusionary line between two adjacent unit strips of recording field is regarded as a problem.
For example, where recording of a unit strip of recording field is completed through four recording runs by a recording head with 64 nozzles aligned to record at a recording resolution of 360 dpi, the width of each unit strip of recording field in the recording medium advancement direction is approximately 1.13 mm (16 nozzlesxc3x9725.4 mm/360 dpi), whereas where each unit strip of recording field is completed through four recording runs, the width of each unit strip of recording field is approximately 4.52 mm (64 nozzlesxc3x9725.4 mm/360 dpi). Therefore, an illusionary line in the second case is easier for human vision to detect; the illusionary line is one of the problems which occur as an attempt is made to improve image quality.
The smaller the interval, the less visible are the illusionary lines. Therefore, in order to make illusionary lines less visible while using a conventional multi-scan recording method, it is possible to increase the number of recording runs used to complete a unit stripe of recording field so that the width of each sub-strip in a unit strip of recording field becomes smaller. However, simply increasing the number of recording runs reduces recording speed, despite the substantial number of nozzles added to increase recording speed. In other words, with the use of a conventional multi-scan recording method, it is impossible to increase both image quality and recording speed of the same time.
Thus, the present invention was made by addressing the ink draw, which is the essential cause of the illusionary line, and its object is to prevent the ink draw itself, or to reduce the magnitude of the aforementioned problem, or the illusionary line, by controlling the interval of the illusionary lines. In other words, the primary object of the present invention is to provide a recording method and a recording apparatus, which can be used in combination to make the illusionary line, which occurs at the border of two adjacent sub-strips in each unit strip of recording field, hard to recognize by reducing the interval of the illusionary lines, and also to effectively utilize the characteristic of a conventional multi-scan recording method so that the ink draw, which is the essential cause of the appearance of the illusionary lines, can be controlled within each unit strip of recording field.
The present invention which accomplishes the above described object is embodied as an ink jet recording apparatus comprising: a recording head, which has a plurality of recording elements aligned in the first direction, that is, the direction perpendicular to the raster direction, and is moved relative to recording medium in the second direction different from the first direction to record on the recording medium; a recording head moving means, which moves the recording head relative to the recording medium in the aforementioned second direction; and a recording head controlling means which controls the movement of the recording head. It is characterized by the following features. That is, the plurality of recording elements of the recording head are grouped into a plurality of sets comprising consecutively placed recording elements. In a recording operation, the recording head is moved relative to the recording medium in the second direction while being moved relative to the recording medium in the first direction after each movement in the second direction. During the first recording run of the recording head loin the second direction, a plurality of recorded strips of recording field, which are separated from the adjacent recorded strips by an unrecorded strip of recording field are produced, and during the immediately following recording run, recording is made across the plurality of unrecorded strips of recording field.
Further, the present invention is also embodied as an ink jet recording method in which a recording head comprising a plurality of recording elements aligned in the first direction, that is, the direction perpendicular to the raster direction, and which records an image by moving the recording head relative to recording medium in the second direction different from the first direction. It is characterized by the following features. That is, in a recording operation, the recording head is moved relative to the recording medium in the second direction while being moved relative to the recording medium in the first direction after each movement in the second direction. During the first recording run of the recording head in the second direction, a plurality of recorded strips of recording field, which are separated from the adjacent recorded strips by an unrecorded strip of recording field, and during the immediately following recording run, recording is made across the plurality of unrecorded strips of recording field.
The present invention is also embodied as an ink jet recording apparatus comprises: a recording head, which has a plurality of recording elements aligned in the first direction, that is, the direction perpendicular to the raster direction, and is moved relative to recording medium in the second direction different from the first direction to record on the recording medium; a means for dividing each unit strip of recording field correspondent to a predetermined number of rasters, into a plurality of sub-strips which extends in the raster direction; a mask selecting means which selects a dot density thinning mask for each sub-strip; a recording controlling means which records across each unit strip of recording field through a plurality of recording runs, each of which is made corresponding to different sub-strips while using a dot density thinning mask different from the masks used for the other runs. It is characterized by the following features. That is, in a recording operation by this recording apparatus, a unit strip of recording field, which can be recorded through a single recording run by the recording head in the second direction different from the first direction, if all the recording elements are activated, is recorded through a plurality of recording runs, each of which is made with the use of a dot density thinning mask different from the dot density thinning mask for the other runs, is recorded so that recording of each sub-strip of recording field extending in the second direction, that is, the raster direction, is completed through two recording runs, each of which uses a set of recording elements different from the set of recording elements used for other runs, and that the controlling means controls the recording head so that a set of a plurality of alternately selected sub-strips of recording field is recorded through the same recording run.
Further, the present invention can provide an ink jet recording method, which is usable with a recording head which comprises a recording head, comprising a plurality of recording elements aligned in the first direction, that is, the direction perpendicular to the raster direction, and is moved relative to recording medium in the second direction different from the first direction to record on the recording medium; and comprises: a step in which a unit strip of recording field is divided into a plurality of sub-strips, each of which extends in the raster direction, and corresponds to a plurality of rasters; a process in which a dot density thinning mask is selected for each sub-strip of recording field; and a process in which each sub-strip of recording field is recorded through a plurality of recording runs made while using a dot density thinning mask. This method is characterized by the following features. That is, in a recording operation, a set of nonadjacent sub-strips of recording field are recorded through the same recording run in the second direction.
According to the above described structure, the interval of the xe2x80x9cillusionary linexe2x80x9d created between two adjacent sub-strips of recording field as the recording of each sub-strip is completed, can be made smaller by reducing the number of rasters necessary to complete the recording of each sub-strip. Further, regarding each sub-strip of recording field, there are the same effects as those produced by a conventional multi-scan recording method. That is, since a unit strip of recording field is recorded with the use of a plurality of sets of nozzles different from the nozzles in the other sets, it is possible to prevent inaccuracy in the ejection direction of each nozzle from negatively affecting image quality. Further, recording is made in such a manner that after a sub-strip of recording field is completed, the recording of a sub-strip of recording field adjacent to the completed sub-strip is started, the magnitude of the ink draw which is the essential cause of the xe2x80x9cillusionary linesxe2x80x9d is smaller. In this specification of the present invention, a word xe2x80x9crasterxe2x80x9d means a strip of recording field, the recording of which can be completed by moving a single recording element (nozzle) in the main scanning direction while ejecting ink.
While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.